A person has died after being hit by a Eurostar train on the tracks.
The horrific accident on Monday afternoon caused the instant death of the victim, who has not yet been identified.
Eurostar confirmed that trains from London, Paris, Brussels and other stations were all subject to cancellations and long delays following the accident.
The company is advising people not to travel unless it's absolutely necessary.
"It happened at around 3.50pm," a spokesman for SNCF, the French national railway, said.
"A Eurostar train hit the intruder at Steewerck, close to Lille.
"All that's certain is that the intruder died.
"There were 600 passengers on board the train and they were all stranded.
"All Eurostar services have been affected."
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The train finally stopped at La Croix au Bac, closer to Lille, where efforts were made to deal with the aftermath of the accident.
Local emergency services were alerted and made their way to the scene.
By 5.30pm a secure area was full of medics, ambulance workers and police investigators.
The incident came as thousands tried to return home from the Easter Bank Holiday weekend and massive queues built up everywhere.
One Twitter user trying to get back home said: "I’m stranded in Paris with elderly parents we need to get back to London.
"Wednesday evening at the earliest is not acceptable."
Another said: "Can you advise me what to do - I'm stuck in Brussels."
Many shared photographs on social media while complaining of queues building up amid the delays and cancelled services.
Some pictures showed crowds gathered at London's St Pancras International.
A tweet by Eurostar says: "Due to a fatality near Lille this afternoon, all trains are subject to long delays.
"We currently advise not to travel unless absolutely necessary.
"To exchange your ticket for free to a different date, or to request a refund please see https://bddy.me/2ZokSVy."
Eurostar also said: "Delays to all remaining trains this evening are expected.
"We are very sorry for the inconvenience caused to our passengers."
Earlier this afternoon Eurostar had tweeted: "We are very sorry that train ES9028 (LON 13:31 > PAR 16:47) is currently stopped on the line, following a fatality near Lille."